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Full text of "Text Book Of Mechanical Engineering"

582

Radiation and Conduction.

both have equal temperatures.    Such transference may occur by
radiation, conduction, or convection.

Radiation is the passage of heat between substances not in
contact, without at the same time raising the temperature of the
intervening medium. Thus a fire may heat surrounding solids,
and the air receive its heat from the solids in turn. To explain
radiant heat, a fluid of infinite tenuity is imagined, called the
Ether, filling space and the interstices of matter, and transmitting
radiant heat, by wave motion, without increasing molecular
motion. If, however, the undulations be arrested, the energy
is absorbed as molecular motion, and becomes apparent in the
arresting body as heat. Radiation is an aid to heat dispersion,
as in heating apparatus, but a disadvantage with boilers and
steam cylinders, there causing loss. Good radiators must
therefore be adopted in the former, and bad ones for the
latter case. Good radiators are good absorbers, to an equal
degree, and reflecting power is the exact inverse of radiating
power.

RELATIVE VALUE OF RADIATORS,

Substance.

Lampblack or soot
Cast iron, polished
Wrought iron, polished
Steel, polished ...
Brass, polished ...
Copper, polished...
Silver, polished ...

Relative
Radiating Value.

100

26

23
18

7

*    5
3

Conduction is the transfer of heat by contact, molecular
motion being then directly caused. Heat is thus transmitted
through the thickness of a furnace tube. There are good and
bad conductors, the former being chosen for fireboxes, other
properties being suitable. (See Appendix /Z, /. 876,)